The present invention relates to a process for preparing polytetrafluoroethylene (hereinafter referred to as "PTFE") granular powder containing a filler and PTFE granular powder prepared by the process.
The molding powder of PTFE is a PTFE powder prepared by finely dividing coarse particles obtained by suspension polymerization, and it has been used for molding by means of compression molding or ram extrusion molding. The particle size of the finely divided PTFE particles (primary particle size) is 5 .mu.m at the smallest and up to about 1,000 .mu.m, usually has an average particle size of not more than 100 .mu.m. The PTFE molding powder into which a hydrophilic or semi-hydrophilic filler is uniformly incorporated also has been used as a filler-containing PTFE molding powder having an improved properties such as abrasion resistance and hardness in comparison with those of the PTFE powder.
It is possible to uniformly incorporate the filler to the PTFE molding powder by means of a special mixing machine. Recently in advance with molding automation, an agglomerated granular powder tends to be used as a filler-containing molding powder having an improved handling property, especially powder flowability, and a high bulk density.
The method for agglomerating and granulating is roughly classified into a dry mixing method or a wet mixing method. The former is a method using no water, and the latter is a method using water. As the typical example of the latter there is known a method in which a mixture of the PTFE powder, a filler and a water-insoluble organic liquid is agitated in water.
The wet mixing method is superior to the dry mixing method because an automation of granulation process can be relatively easy, though a separation step and a drying step after the aqueous treatment are required.
A difficulty has been encountered in the uniform mixing of the PTFE powder with a hydrophilic or semi-hydrophilic filler like a glass powder, because the hydrophilic or semi-hydrophilic filler tends to transfer easily into an aqueous phase. As a result, an agglomerated PTFE granular powder containing all of the hydrophilic or semi-hydrophilic filler used cannot be obtained, and a portion of the filler remains in the treating water. The phenomenon is called as "separation of filler". Also the filler in the granular powder is apt to come off from the obtained granular powder during handling.
In order to solve these problems, there is employed a process in which, prior to the mixing in water, the hydrophilic or semi-hydrophilic filler is previously subjected to hydrophobic surface treatment, whereby lowering the surface activity near the surface activity of the PTFE powder, or a process in which the mixing is carried out in the water-organic liquid medium to which a certain compound having the above activity-lowering effect is added at the mixing.
Among the processes, as processes in which a silicon-containing compound is used, there have hitherto been known processes as mentioned below.
(a) A process in which an amino functional organosilane and/or a water-soluble silicone resin is added into a water-organic liquid medium (Japanese Examined Patent Publication No. 47269/1978, No. 40099/1979). PA1 (b) A process in which a silicone resin and melt processible TFE copolymer particles are present in the system (Japanese Examined Patent Publication No. 7164/1982). PA1 (c) A process in which a filler surface-treated with an aminosilane compound is mixed in the presence of a nonionic surface active agent (Japanese Examined Patent Publication No. 21694/1985). PA1 (d) A process in which a filler surface treated with a perfluoroalkylsilane is used (Japanese Unexamined Patent Publication No. 139628/1989).
It is extremely insufficient to prevent the separation of filler according to the method (a). In the method (b), there is a disadvantage that a heat resistance of a PTFE powder is lowered. The methods of (c) and (d) in which a filler is previously surface-treated do not have the above-mentioned disadvantages of (a) or (b). In the method (c) using an aminosilane, however, the obtained molded article is colored in a central part after sintering. Also in the method (d), though the obtained molded article is endowed with excellent electric characteristics, there is a disadvantage of coloration similarly to the method (c).
According to the present invention, there can be provided a process for decreasing the separation of filler and preparing a PTFE granular powder in which a filler is uniformly dispersed. The obtained granular powder can give a colorless molded article.